By Maureen N. Harbourt

EPA is required by Section 109(d) the Clean Air Act to review the adequacy of each National Ambient Air Quality Standard (“NAAQS”) every five years to determine if new scientific evidence justifies a change to the standard.  The current primary[i] NAAQS for nitrogen dioxide (“NO2”) is 53 ppb annual mean and 100 ppb NO2 as 98th percentile of 1-hour daily maximum concentrations, averaged over 3 years.  The annual average was first adopted in 1971, and was not changed during (overdue) reviews completed in 1985 and 1996.  In the next completed review in 2010, EPA added the 1 hour NO2 NAAQS to the standard based on a conclusion that the annual standard alone was not protective enough due to potential health impacts associated with short term exposures.[ii]  The 2010 review also indicated that there was a lack of data concerning near roadway exposures, which was of concern given that 34% of NO2 emissions are estimated to be generated from roadway vehicles.  Thus, the 2010 review led to EPA requiring states to install near-roadway monitors in urban areas during the 2014-2017 period.

EPA has just completed a final Policy Assessment reviewing the adequacy of the 2010 NAAQS and has concluded that no change to the existing standard is recommended.  82 Fed. Reg. 17947, April 14, 2017. The EPA Clean Air Science Advisory Committee (“CASAC”) also recommended no change to the standard.  The EPA’s Policy Assessment indicated that the additional roadway monitors installed as a result of the 2010 NAAQS rule have not been gathering data for a sufficient period (only 1-2 years) to fully evaluate such information, although the data that was available showed higher NOx concentrations near roadways than at nearby non-roadway monitors.[iii]  The Policy Assessment is the last step of the periodic NAAQS review process before any final EPA decision to revise or not revise the existing standards. It considers the Integrated Science Assessment, the Risk/Exposure Assessment, and the advice of the CASAC.


[i] The primary NAAQS are set at a level to protect human health with an adequate margin of safety.  A secondary NAAQS is set at a level to protect human welfare, including decreased visibility and damage to animals, crops, vegetation, and buildings.  The secondary NAAQS for NO2 is currently equivalent to the annual primary standard (53 ppb annual mean).  EPA has recently completed an integrated science assessment for the secondary standards for NO2, sulphur oxides and particulate matter and has requested that the CASAC review that assessment.  82 Fed. Reg. 15701, March 30, 2017.

[ii] The EPA’s decision to add the 1-hour NO2 NAAQS was upheld in American Petroleum Institute v. Environmental Protection Agency, 684 F.3d 1342 (D.C. Cir. 2012), cert. den. 133 S.Ct. 1724 (2013).

[iii] The full EPA Policy Assessment is available here.